The present invention relates generally to a phase-locked loop circuit and, more particularly, to a phase-locked loop circuit which is suitable for a frequency synthesizer for obtaining desired oscillating frequencies.
In general, a phase-locked loop circuit comprises a reference oscillator including a crystal resonator, a phase comparator, a low-pass filter and a voltage controlled oscillator. The phase comparator detects the difference in phase between a reference oscillating signal generated from the reference oscillator and a oscillating signal generated from the voltage controlled oscillator. The output voltage of the phase comparator is smoothed by the low-pass filter, and the smoothed voltage serves as a control voltage for the voltage controlled oscillator and, accordingly, the phase of the oscillating signal from the voltage controlled oscillator becomes the same as that of the reference oscillating signal.
In order to obtain a desired oscillating frequency, a l/n frequency divider is provided between the voltage controlled oscillator and the phase comparator in the above-mentioned phase-locked loop circuit. The l/n frequency divider is usually called a programmable frequency divider, since the frequency division ratio of the divider is controlled by an external signal. In this case, an output frequency f.sub.s is n times a reference oscillating frequency f.sub.r. That is, such a phase-locked loop circuit serves as a frequency synthesizer and, accordingly, an output signal of the voltage controlled oscillator is used for controlling a local oscillator of a receiver, such as a radio, a reference oscillator of a transceiver or the like. In such a phase-locked loop circuit, in order to prevent noise from being generated in the radio or the transceiver when the phase-locked loop is out of lock, a muting circuit is provided in the radio or the transceiver.
In the prior art, in order to control the muting circuit, a lock detector is connected to the phase comparator and, in addition, an integrator circuit is connected to the lock detector. The integrator circuit converts a pulsed (rectangular wave) lock detecting signal into a direct current lock detecting signal which is used for controlling the muting circuit.
However, in the above-mentioned prior art, since an integrator circuit is used for converting the pulsed lock detecting signal into a direct current lock detecting signal, a semiconductor phase-locked loop device adapted for generating a direct current lock detecting signal, in which the reference oscillator, the programmable frequency divider, the phase comparator and the lock detector are integrated, requires external components. In addition, such a semiconductor device requires a large number of external terminals. Further, the rising and falling speed of the direct current lock detecting signal is slow and unstable.